The phenomenon of interference in insulating glass – (the so-called rainbow on the glass).
Optical effect – complaints are sometimes received regarding the appearance of a “rainbow” of colored patterns in insulating glass units. They are not caused by defects in the glass or dirt on the glass, but are solely an optical effect caused by light waves interfering with each other when they pass through or are reflected from the insulating glass. There are usually two types of fringes known as Brewster’s Fringe and Newton’s Fringe. Brewster striations are more visible in float glass sets than in drawn glass sets. The reason for this is that float glass has optically almost flat surfaces and the glass thickness is more uniform. Typically, these patterns are only visible when the set is viewed from an angle. In the case of Newton’s Stripes, the patterns are only seen in the center of the set and are circular or elliptical in shape. Their presence indicates a faulty set that should be repaired or replaced.
What are Brewster’s bands?
BREWSTER STRIPES – These may appear in a set where the two panes are almost the same thickness. The difference in thickness between the two panes must be very small and approximately the same as the average wavelength of visible light, i.e. 0.0005 mm. Millions of light waves pass through the insulating glass or are reflected from it. The reason that Brewster’s Stripes are observed more often in float glass sets than in drawn glass sets is because float glass, which is a much better quality product, is very flat and has relatively small differences in thickness. However, as has already been said, a necessary condition for the occurrence of Brewster’s Stripes is the presence of two panes of almost equal thickness in an insulating glass unit, their difference in thickness should be approximately 0.00035 to 0.0007. This condition, although rare, is more often met for float glass than for drawn glass. In addition, drawn glass contains defects that mask Brewster’s striations that may appear.
What are Newton’s Fringes?
This optical effect only occurs in defective insulating glass units when two panes of glass touch (one to the other) or almost touch in the center of the insulating glass unit. This optical effect is a series of concentric colored rings in the center which is the point of contact or close contact between the two panes. The rings are approximately circular or elliptical. The reason for Newton’s fringes is again interference, in which light waves have different path lengths when passing through a faulty thermal insulation assembly or are reflected from it. In this case, however, it is not the almost equal thickness of the panes that causes the waves to shift out of phase, but the interference now occurs within a thin, wedge-shaped film of air adjacent to the point of contact, or almost contact, of the two panes.
